1. Field of the Invention
The present invention relates to a magnetic sheet for use in a radio frequency identification (RFID) antenna, a method of manufacturing the same, and a radio frequency identification (RFID) antenna using the magnetic sheet. More particularly, the present invention relates to a magnetic sheet for use in a radio frequency identification (RFID) antenna, a method of manufacturing the same, and a radio frequency identification (RFID) antenna using the magnetic sheet, in which the magnetic sheet is a mono-layer structure which is made of alloy powder including at least one amorphous alloy, or a multilayer structure which is formed by laminating an amorphous alloy ribbon made of an amorphous alloy between magnetic sheet layers and compression-molding the same.
2. Description of the Related Art
Recently, a radio frequency identification (RFID) system communicating data between a transponder including an integrated circuit (IC) chip and a reader/writer, or between a transponder and a reader is being widely spread. Since the RFID system communicates data using a respective antenna which is included in a transponder and a reader/writer, the transponder and the reader/writer do not need to contact each other to communicate data therebetween. Thus, although the transponder is far from the reader/writer by several centimeters to several tens of centimeters, they can communicate each other. Since the radio frequency identification (RFID) system is not sensitive to contamination or static electricity, it is being used in various fields including a production control in a factory, the administration of physical distribution, an inventory control, and an entrance and exit control.
For example, mobile phones employ a radio frequency identification (RFID) system. Generally, the mobile phone includes a main body and a battery unit which are separably combined with each other. In the case of the mobile phone using the RFID system, the main body includes an integrated circuit (IC) chip on a surface which contacts the battery unit, and the battery unit includes a tag antenna on the reverse surface which does not contact the main body and a battery on the surface which contacts the main body.
The tag antenna and the battery forming the battery unit may be incorporated in a plastic case. Here, the IC chip of the main body and the tag antenna of the battery unit play a role of a transponder of the RFID system, and these communicate data with a reader positioned at the outside of the mobile phone. In more detail, in the case of a RFID mobile phone chiefly using the frequency of 135 kHz or less or 13.56 MHz, especially, an inductive mode, a reader antenna produces a sine wave which is a radio frequency electromagnetic wave, using mainly using the frequency of 13.56 MHz. Accordingly, if the energy is delivered to the tag antenna, that is, toward the transponder antenna through the reader antenna, the transponder is activated and the reader receives data from the tag antenna.
The RFID system of the frequency region as described above, achieves mutual communications by an inductive electromagnetic coupling of a transformer mode. For this purpose, the tag antenna may be formed of a rectangular planar spiral inductor. In this case, communications are achieved by a LC resonance of an antenna, in which a resonant frequency is determined by the inductance of an inductor and the capacitance of a capacitor. Generally it is important that a transponder is designed to employ a parallel resonance circuit, in which it is designed to obtain the maximum impedance at the resonant frequency by the LC resonance in order to obtain the maximum voltage with the minimum current.
FIG. 1 illustrates a radio frequency identification (RFID) system including a reader 100 and a transponder 200 between which data communications are achieved by an inductive coupling mode. The reader 100 includes an oscillator 1 for producing an electromagnetic wave of 13.56 MHz, a capacitor 2, a resistor 3, and a coil 8. In the transponder 200, a magnetic field 6 generated from the coil 8 in the reader 100 reaches a radio frequency identification (RFID) antenna coil 7 attached to the transponder 200 and thus voltage is induced across the coil 7. The induced voltage passes through the capacitor 2 connected in parallel with the RFID antenna coil 7 and a diode 4 connected in series with the RFID antenna coil 7 and thus direct-current (DC) voltage is supplied to a radio frequency identification (RFID) chip 5.
As the RFID chip 5 is supplied with the DC voltage, the transponder 200 is activated to transfer ID information stored in the RFID chip 5 to the reader 100 through the RFID antenna coil 7.
In this case, the voltage induced in the tag antenna is determined by the Faraday's law and Lenz's law. Therefore, it becomes more advantageous to obtain a higher voltage signal, as a more amount of magnetic flux interlink with the transponder antenna coil. The amount of the magnetic flux becomes larger, as an amount of a soft magnetic material included in the transponder antenna coil becomes larger and the magnetic permeability of the soft magnetic material becomes higher. Particularly, since the RFID system performs contactless data communications intrinsically, it is necessary to have an absorber sheet which is made of a magnetic material having a high magnetic permeability in order to focus a radio frequency electromagnetic wave which is made in the reader antenna on the tag antenna.
In the case of an antenna coil for use in a generally given transponder, an inductance of the antenna coil is in proportion to the magnetic permeability of the magnetic material. In a communications process, as the inductance of the transponder antenna becomes high, higher voltage is induced proportionally to the inductance of the transponder antenna. Accordingly, if a magnetic material having a higher magnetic permeability is employed as a material of the absorber sheet for the tag antenna, a data communication distance is increased and an error probability of data can be lowered.
The conventional magnetic material for use in an absorber sheet for a mobile phone RFID antenna is fabricated into a sheet form which is formed by mixing the ferrite including Mn—Zn group oxide or Ni—Zn group oxide together with resin. In the conventional technology, it is very difficult to tune the LC resonance circuit since an inductance deviation of the transponder antenna is serious due to the non-uniformity of the thickness of these ferrites. Moreover, the non-uniformity exceeding the error range of the thickness may cause difficulty of an installation when a magnetic sheet is attached in a battery pack space, and an error occurs in the process of the formation and transmission of binary code data during the mutual transmission process of data, to thereby make a very high inferiority ratio, as well.
Moreover, the recent mobile phone adopts high functions such as a game, a video communication, an Internet function addition, and a camera. Accordingly, an amount of the power consumption increases. As a result, it is necessary to have the volume of a battery as large as possible. Furthermore, miniaturization and thin shaping of a device is being progressed. But in the case that the absorber sheet for the RFID antenna made of ferrite is in the thickness of 0.35 mm or less, the inductance for the RFID communications of the transponder antenna is low. In this case, the communications distance is excessively shortened, that is, the quality of performance is not implemented. As a result, development of a new material is acutely needed.
In the meantime, the Moly Permalloy Powder (MPP) containing permalloy or molybdenum (Mo) having an excellent soft magnetic property is used as a magnetic material of an absorber for a mobile phone RFID antenna. The Permalloy or MPP is excellent in view of a soft magnetic property in comparison with ferrite and thus is used as the absorber sheet for the mobile phone RFID antenna even in the thickness of 0.2 mm. However, since the powder manufactured in the form of a spherical shape should be pulverized into the nano size and made to be flat, the manufacturing process is complicated and the original material is expansive.
As described above, in the conventional technology, in the case of the ferrite or the Permalloy containing the molybdenum used as the magnetic material, these are formed of the crystalline of the spherical shape irrespective of the processing condition. Therefore, complicated processes such as a micro-powdered process and then a flatness process should be undergone in order to manufacture the absorber sheet of the thin thickness of 0.2 mm or less using the ferrite or the Permalloy containing the molybdenum. Moreover, since these conventional materials have the problem that an effective cross-sectional area of the material representing the magnetic property cannot be secured as the thickness of the conventional materials is thinner, it has a limitation in heightening the inductance necessary for securing the distance of the RFID communications. Also, since a thickness of the conventional materials is uneven during manufacturing, the deviation of inductance becomes serious.
Besides, U.S. Pat. No. 6,887,412 describes a composite magnetic sheet and method of producing the same capable of suppressing electromagnetic interference. The composite magnetic sheet includes composite magnetic bodies having flat soft powder subjected to annealing to be free from stress strain and a binder. The composite magnetic sheet is pressed by the press or the rolling mill having the rolls in a direction perpendicular to the plane of the sheet. Further, multiple composite magnetic sheets are stacked by sandwiching Al plate or wire.
However, the magnetic permeability of the magnetic sheet is not high even in the thin thickness below 0.35 mm. While some of the objects and the structural components of U.S. Pat. No. 6,887,412, at first appearance, have similarities with the present invention, U.S. Pat. No. 6,887,412 differs in amorphous alloy powder composition and structural respects. These differences, which will be described in more detail hereinafter, are essential for the effective use of the invention and which admit of the advantages that are not available with the prior devices.